Chapter 1 Longitudinal Data Analysis
Themostcommonmedicalresearch designsis a \pre-post"studyinwhich a singlebaselinehealthstatusmeasurement is obtained,aninterven-tionis administered,anda singlefollow-upmeasurement is theoutcomemeasurement canbe as-sociatedwiththechangein example,if somesubjectsaregivenplacebo whileothersaregivenanactive drug,thetwogroupscanbe comparedto seeif thechangein theoutcomeis di erent forthosesubjectswhoareactivelytreatedas comparedto viewed as thesimplestformof a prospective nition: Alongitudinalstudyrefersto aninvestigationwherepartici-pant outcomesandpossiblytreatments or exposuresarecollectedat longitudinalstudygenerallyyieldsmultiple or \repeated"measurementsoneach example,HIVpatients may be followedover timeandmonthlymeasuressuch as CD4counts,or viralloadarecollectedto charac-terizeimmunestatusanddiseaseburde nrespectively.
tional study of the e ects of a drug on speci c indicators of health, a patient’s current health status may in uence the drug exposure or ... Although scienti c interest often focuses on the mean response as a func-tion of covariates such as treatment and time, proper statistical inference
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